Hydroxylammonium nitrate is useful as a reducing agent in re-processing spent nuclear fuel, as an energetic oxidizer salt for use in liquid gun propellants, hybrid rocket motors, solution propellants and as a reagent for preparing various industrial, specialty and pharmaceutical chemicals.
It has previously been reported in the literature that commercially available solutions of hydroxylammonium nitrate may be quite dilute, oftentimes being shipped in inert containers at a maximum concentration of 24 percent.
Various methods for preparing hydroxylammonium nitrate have been proposed over the years. These processes suffer from various drawbacks from a commercial, safety and/or technical perspective.
Several of the known processes for preparing hydroxylammonium nitrate have inadequate product yields, have impurity laden products, and/or present problems in accurately controlling the concentration of the desired salt. For instance, one known electrolytic process for making hydroxylammonium nitrate has the undesirable environmental characteristic of utilizing significant amounts of mercury. As a consequence, the hydroxylammonium nitrate may contain a residual amount of mercury, and extensive mercury contamination can occur if the electrolysis cell should rupture. The process is quite expensive and it has been said that it is difficult to obtain consistent concentrations of the desired product. Further, the process apparently does not produce hydroxylammonium nitrate in sufficient concentration for direct use in making liquid gun propellants, and requires concentrating the hydroxylammonium nitrate solution.
An electrochemical process using a mercury cathode in an electrolysis cell is described in French Patent 2,602,802 (1988).
Earlier efforts involving electrodialysis with hydroxylammonium sulfate or its chloride and nitric acid using an ion exchange membrane to produce hydroxylammonium nitrate are described in French Patent Application 2,243,904 and in Ind. Eng. Chem. Process Des. Dev. 20:361 (1981). In this method, hydroxylammonium nitrate is produced from hydroxylammonium sulfate or hydrochloride by an electrolysis method which uses cation and anion exchange resins and a double decomposition reaction. Four different types of fluids are passed through four chambers of an electrodialysis tank having the chambers in one block but partitioned alternately by cation exchange membranes and anion exchange membranes. The hydroxylammonium nitrate is drawn out of one of the four chambers. The process is complicated, and commercially not very attractive because it requires four units of fluid circulating pipes, direct current power sources, and auxiliary facilities, and productivity is low. Further, the available membrane surface area is limited.
A process for continuously converting a hydroxylammonium salt, such as the sulfate salt, to another hydroxylammonium salt, such as hydroxylammonium nitrate or hydroxylammonium formate, by counter-current liquid extraction, using a so-called cationic solvent is described in French Patent 2,206,270 (1974).
Conversion of hydroxylammonium sulfate into hydroxylammonium nitrate has been proposed using a cation exchange resin. Ind. Eng. Chem Process Des. Dev. 16:220 (1977). In this method, a sulfate is converted into a corresponding nitrate using a cation exchange resin in which the hydroxylammonium ion in the hydroxylammonium sulfate is carried on a cation exchange resin and then eluted with a nitric acid solution. This method is a quite complicated batch process and there is a danger of explosion by the reaction between the cation exchange resin and the nitric acid. Chem. Eng. Nov. 17:271 (1980). The hydroxylammonium nitrate solution produced is extremely dilute.
Hydroxylamine nitrate can be produced by combining NO and H.sub.2 gases in the presence of a platinum catalyst, German Offenlegunschrift 2,100,036, or by hydrogenating nitric acid in the presence of a special palladium catalyst, Dutch patent application 7,009,685. Another process for preparing a hydroxylammonium salt by the catalytic reduction of nitrogen monoxide with hydrogen at elevated temperature in a dilute aqueous solution of a mineral acid and in the presence, in suspension, of a particular supported platinum catalyst is disclosed in Europe 287,952 (1988). The processes for producing hydroxylammonium nitrate wherein nitrous oxide or nitric acid is hydrogenated in the presence of the specified catalyst requires the use of dangerous hydrogen and, in the other case, nitrous oxide. The processes require a special catalyst which must be periodically replaced or regenerated. Catalyst regeneration is complicated and expensive.
The stoichiometric reaction of a boiling saturated barium nitrate solution with hydroxylammonium sulfate yields a dilute solution of hydroxylammonium nitrate. The maximum concentration of hydroxylammonium nitrate obtained is about 20%. However, concerns about product stability often mean the reaction is conducted at lower temperatures which results in a reduced product concentrations on the order of 15%. Further, unless the barium nitrate salt is dissolved prior to admixture with hydroxylammonium sulfate, results are erratic and considerable amounts of barium salt remain unreacted.
Another barium nitrate based process for making hydroxylammonium nitrate is described in U.S. Pat. No. 4,066,736. As the process is described, a hydroxylammonium salt, such as hydroxylammonium sulfate, is slowly added to a well agitated slurry of barium nitrate whereby the hydroxylammonium salt goes into solution and reacts with the dissolved barium salt but does not directly contact the slurried barium nitrate. Effective agitation must be maintained, otherwise direct contact between the slurried barium nitrate and the hydroxylammonium salt occurs whereby the once soluble slurried salt will receive an insoluble barium sulfate coating which causes the reaction to terminate prematurely. The insoluble by-product, barium sulfate, must be separated from the hydroxylammonium nitrate. This may present handling, separation and disposal problems.
Europe 108,294 (I984) describes preparation from solid hydroxylammonium sulfate of alcoholic hydroxylamine solutions, and of oximes, hydroxamic acids and other hydroxylammonium salts via alcoholic hydroxylamine solutions. Among the methods disclosed is one for preparing hydroxylammonium nitrate which involves preparing and stirring a methanolic solution of sodium methoxide and hydroxylamine sulfate in an ice bath, filtering the thus obtained slurry, mixing the clear methanolic filtrate with cake-wash and with concentrated H.sub.2 SO.sub.4 to pH 8.0, filtering thus produced white solid, and adding conc. HNO.sub.3, while chilling and stirring, to the filtrate whereby hydroxylammonium nitrate in methanolic solution is said to be obtained.
Another dialysis method, which is described in Europe 266,059 (1988), involves providing a dialysis unit comprising a tank divided into chambers by a cation exchange resin, such as styrene and divinyl benzene, placing an aqueous solution of hydroxylamine sulfate and a solution of nitric acid in adjacent chambers, and allowing the hydroxylammonium ions and hydrogen ions to pass through the membrane whereby the nitric acid is converted to the desired hydroxylamine nitrate.
A method for preparing hydroxylamine salts is described in U.S. Pat. No. 4,956,168 in which a slurry of hydroxylamine sulfate in alcohol at a temperature not exceeding 65.degree. C. is prepared, a hydroxylamine-alcohol solution and ammonium sulfate are then prepared by high-shear mixing ammonia with the slurry at .ltoreq.65.degree. C., the ammonium sulfate is removed by filtration, the solution is agitated, and nitric acid is admixed under vigorous agitation with the solution at about .ltoreq.50.degree. C. and preferably not below 20.degree. C., and the desired hydroxylamine salt is recovered.
Low concentrations of aqueous hydroxylammonium nitrate solutions may pose obstacles in the effective use of hydroxylammonium nitrate in certain applications such as, for instance, a liquid gun propellant and in making pharmaceutical or other specialty chemicals. Accordingly, efforts to find suitable means for concentrating solutions of hydroxylammonium nitrate have been made. However, it has been stated that concentration of solutions containing hydroxylammonium nitrate by techniques which require heating, such as distillation or evaporation, are not desired because decomposition of the product may result.
One effort to provide a suitable process for concentrating dilute aqueous solutions of hydroxylammonium salts, including hydroxylammonium nitrate, is disclosed in U.S. Pat. No. 4,851,125. This processes involves concentrating the hydroxylammonium nitrate salt solution through contact with a membrane having a sorption side and a desorption side, and comprises contacting the dilute salt solution with the sorption side of a membrane to sorb the solvent and permit the solvent to flow through the membrane to the desorption side, and desorbing the solvent from the desorption side of the membrane. This process does not address the underlying problem of providing an economical, environmentally acceptable and technically facile process for manufacturing hydroxylammonium nitrate.
Accordingly, there has been a long standing need for a less capital intensive and facile process for producing hydroxylammonium nitrate solutions sufficiently pure and concentrated for direct use in energetic compositions and in syntheses of pharmaceutical or specialty chemicals.